Earlier on Sunday, a train presumed to belong to the North Korean leader was spotted at a station in Wonsan amid speculation about the leader's health

Amid widespread speculation over the health and absence of North Korea's Kim Jong-un, South Korea has stepped in to refute reports of the 36-year-old leader's poor health, saying he is "alive and well."

Earlier on Sunday, South Korea's Yonhap news agency reported via a US monitor citing commercial satellite imagery that a train presumed to belong to the North Korean leader was spotted at a station in Wonsan even as some US outlets speculated that he had succumbed following a botched heart operation

'No suspicious movements'

"Kim Jong-un is alive and well. He has been staying in the Wonsan area since April 13. No suspicious movements have so far been detected," Moon Chung-in, the top foreign policy adviser to South Korean President Moon Jae-in, told CNN.

Last week, Seoul officials had disputed recent media reports about Kim, saying there have been no unusual signs from North Korea. Some said that Kim is presumed to be staying in Wonsan for unspecified reasons.

According to 38 North -- a website devoted to analysis about North Korea, the imagery showed a train "probably belonging to Kim Jong-un parked at the Leadership Railway Station servicing his Wonsan compound since at least April 21."

"The approximately 250-metre long train, although partially covered by the station's roof, can be seen at a railway station reserved for use by the Kim family. It was not present on April 15 but was present on both April 21 and 23," it said.

"The train's presence does not prove the whereabouts of the North Korean leader or indicate anything about his health, but it does lend weight to reports that Kim is staying at an elite area on the country's eastern coast," it added.

The report of Kim's illness started doing rounds following his absence at the Kumsusan Palace of the Sun in Pyongyang on the birthday of late state founder and his grandfather, Kim Il-sung, earlier this month.

Thereafter, CNN intensified the speculation by reporting earlier last week that the US is looking into intelligence that Kim Jong-un is "in grave danger" after surgery. Later on Saturday, other media reports stated that China has dispatched a team of medical doctors and officials to North Korea "to advise on" Kim Jong-un, citing multiple unnamed people familiar with the situation.

North Korea's state media, however, has not made any mention of Kim's public activity for two weeks since he was last seen on April 11 presiding over a major party meeting, though it has reported on his handling of routine state affairs, such as sending diplomatic letters.